Coal-fired flue gas and flue gas generated by burning heavy fuel oil may contain metallic mercury (Hg0) as well as dust, sulfur oxide (SOx), and nitrogen oxide (NOx). In recent years, various proposals have been made on methods and devices for treating the metallic mercury) (Hg0), in combination with a denitration device that reduces NOx and a wet desulfurization device that uses an alkali absorbent as a SOx absorbent.
As a method for treating metallic mercury (Hg0) in flue gas, a method including spraying NH4C1 solution in liquid state into a flue gas duct in the upstream process of a reduction denitration device to supply it into a flue gas duct has been proposed (for example, see Patent Literatures 1and 2). When a NH4C1 solution in liquid state is sprayed into a flue gas duct, NH4C1 is dissociated to produce ammonia (NH3) gas and hydrogen chloride (HC1) gas. NH3 gas functions as a reducing agent while HC1 gas functions as a mercury chlorinating agent (that is, oxidation assistant). Specifically, on a denitration catalyst filled into the reduction denitration device, NH3 is reduced with NOx in the flue gas as represented in the following formula 1, and HC1 is oxidized with Hg0 in the flue gas as represented in the following formula 2. When NH3 is subjected to reduction nitration on a denitration catalyst, the metallic mercury (Hg0)is also oxidized to give water soluble mercury dichloride (HgC12), and then with a wet desulfurization device installed in the downstream, HgC12 is dissolved in a limestone and gypsum slurry as an absorbent, and therefore mercury contained in the flue gas is removed.4NO+4NH3+O2→4N2+6H2O  (1)Hg0+½O2+2HCl→HgCl2+H2O  (2)
Further, with regard to a desulfurization device for absorbing and removing sulfur oxides in flue gas by contacting flue gas with limestone and gypsum slurry of an absorbent, when hyperoxidized state occurs in an absorber, part of mercury oxide (Hg2+), which is water soluble, may turn into insoluble metallic mercury (Hg0) and is discharged from a stack to the outside.
In this regard, when a reduction state occurs in an absorber, part of mercury oxide (Hg2+), which is water soluble, may turn into insoluble metallic mercury (Hg0) and is discharged from a stack to the outside.
As a means for dealing with such problem, having an oxidation state in the absorber can suppress the aforementioned phenomenon (that is, reduction of mercury oxide to metallic mercury and discharge from a stack to the outside).
However, according to conventional methods, the oxidation state may progress excessively, so that it is prone to have a hyperoxidation state, and also sulfur dioxide gas SO2 and selenium compound from flue gas, that are absorbed into an absorbent, are hyperoxidized to yield Se6+, S2O62−, and S2O82−. As they are in stable form and difficult to be processed by waste water processing, it is necessary to prevent in advance their production. To do so, it is required to control the oxidation and reduction state so that a hyperoxidation state is not yielded.
Accordingly, a method of canceling a hyperoxidation state in an absorber is suggested, in which, when the hyperoxidation state is caused and an operation reference value in an absorber exceeds a given range, at least one process of the method for canceling a hyperoxidation state in an absorber that is composed of the followings are performed: automatically adjusting soot and dust removing amount from flue gas taken into a flue gas desulfurization device, automatically adjusting an absorbent slurry circulation amount, reducing an oxidizing air flow rate fed to the absorbers, automatically adjusting an absorbent slurry amount fed to the absorbers, and increasing the absorbent slurry amount fed to a dehydrating unit (Patent Literature 3).